(a) Field of the Invention
The present invention relates to a method for preparing an aromatic carbonate and a reaction apparatus for the same, and particularly to a continuous method for preparing an aromatic carbonate using a heterogeneous catalyst and a reaction apparatus for the same.
(b) Description of the Related Art
Aromatic carbonates are used as starting materials in the preparation of several kinds of isocyanates and aromatic polycarbonates that do not use a very toxic phosgene gas, and they are synthesized mostly by the transesterfication of dialkyl carbonates and aromatic hydroxy compounds. In this reaction, typical reactants are dimethyl carbonate and phenol.
However, such transesterfication is a reversible reaction and its equilibrium constant is very small, and consequently the conversion rate after reaction is very low and the reaction rate is quite slow, and thus it has numerous difficulties in being used in commercial production.
Several attempts have been made to solve such problems, including attempting to increase the reaction rate by improving the performance of a reaction catalyst.
U.S. Pat. No. 4,182,726 disclosed a process of using an AlX3 such as AlCl3, UX3, TiX3, VOX3, VX3, ZnX2, FeX3, and SnX3 class reaction catalysts, wherein X refers to halogen group elements. Also, U.S. Pat. No. 4,045,464 disclosed using Ti class compounds such as titanium tetraphenate or Lewis acids.
Further, U.S. Pat. No. 4,552,704 disclosed Ti and Sn class reaction catalysts such as butyltin oxide hydroxide, U.S. Pat. No. 4,554,110 disclosed polymeric tin compounds as reaction catalysts, and U.S. Pat. No. 4,609,501 disclosed a reaction catalyst mixing at least one Lewis acid and at least one protic acid.
These prior arts can be said to try to improve the reaction rate by the action of the reaction catalyst rather than to improve reactivity by the reaction process, in order to increase efficiency in the preparation of DPC (diphenyl carbonate) from DMC (dimethyl carbonate).
However, all of the catalysts proposed in these patents are homogeneous catalysts, and thus when they are used they should be mixed with reactants in a certain ratio. Also, even though reaction by-products such as methanol, which are generated during reaction, were allowed to be continuously eliminated from the reaction system, the reaction rates shown in the examples were not high.
On the other hand, there have been attempts to use heterogeneous catalysts as reaction catalysts. U.S. Pat. No. 5,354,923 employed a stirring reactor using a catalyst in a powdered form having a surface area of not less than 20 m2/g, and U.S. Pat. No. 5,565,605 applied heterogeneous catalysts such as titanoalumino phosphate for reaction.
However, the heterogeneous catalysts prepared hitherto are in a powdered form and they are subject to reaction and separation processes together with the reactants, and thus they cause a problem of being adhered to the inside walls of pipes and equipment. They are also problematic in their re-use after separation because they co-exist with reaction products having a high boiling point.
Further, there have been attempts to increase efficiency in the preparation of DPC by designing unique reaction processes along with the development of reaction catalysts. U.S. Pat. No. 5,210,268 employed a multi-stage distillation column as a reactor, wherein two reactants having different boiling points were counter-currently contacted by injecting the reactant having a high boiling point to the upper portion of the column together with a homogeneous catalyst and by heating and evaporating the reactant having a low boiling point at the bottom portion of the column to synthesize an aromatic carbonate.
Also, U.S. Pat. No. 5,426,207 disclosed a reaction process using three reactors that were connected in series, and U.S. Pat. No. 5,523,451 disclosed a process of applying multiple bubble columns that were connected in series to a reactor. Also, U.S. Pat. No. 6,057,470 disclosed a reaction process using a reaction distillation method.
However, these processes have numerous difficulties in the separation of catalysts because they use homogeneous catalysts or catalysts in a powdered form, and they also have difficulties in economic aspects because a certain amount of catalyst must be continuously supplied to the reaction system in a continuous operation.